Hi,

I have a 1979 F150 supercab with a 400M engine and some break problems.

First off, thanks everyone for your comments on the proportioning valve. I got that replaced. But a couple of days after I got it fixed my (one month old) master cylinder went again. Replaced that today. But now my break light comes on intermittently Not as much as when the master cylinder was bad. The master cylinder is holding pressure. I also hear a ssssssss noise tha goes away if I lightly touch the breaks while driving. I think I have a leak somewhere in the power break booster. The check valve thing is loose on the Booster unit. Don't know if this means anything, but the one at the parts store was solidly attached.

I think my troubles started when the proporting valve got stuck when I had a break line leak. I tried standing on the breaks to reset the valve as did my mechanic. Maybe we tried with too much force and did some harm. It was right after this that my new master cylinder died and I started hearing the vacuum leak noise.

Tomarrow I will check the pads in front just to be sure it is not them, but my break light is coming on most of the time I stop. So the noise is probably not from there. If I tap the breaks when the light is on the light goes out.

Does this sound like the booster is leaking and is there any definative test. I am going to replace the vacuum hose first, but don't think it is going to help.

Thanks

Jeff

 

First off the correct spelling is : "Brake"

Now, Try this:
With the engine shut-off pump the brake pedal and hold.
Now with your foot still on the brake pedal> Start the engine<
What happens to the pedal ?
Does it stay ? go to the floor ?

Chances are it's the *Check valve*

Maybe buy a haynes or better yet, a ford shop manual ,they have complete diagnosis procedures in them.
Not that we can't remedy your problem here ,just that you would have the book to refer to as you try and figure it out when you're
with your truck.


Dennis
Please Don't Ask Me Any Tough Question As:
"I'm Saving Up MY Memory For When I Develop Alzheimer's"







78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles

 

Well, I swapped out the vacuum hose and it did not make a difference. I ran a system test by pressing the brake and starting the engine. The break pedal went down with light pressure applied. I ran a vacuum check by running the engine without touching the brakes, shut it off and let it sit for a minute and lightly pumped the brakes. The pedal went to the same location with each pump. This indicates that there is a vacuum leak.

Now my question is it most probably the check valve? I can order one and the grommet from Advance Auto and have it in about 5 days. I have not found one in stock at other locations yet. I am a bit afraid of pulling the old one off and in the process messing it up until I can get a replacement.

Now, would having a vacuum leak at the check valve cause the brake light to come on? If the booster is bad, would this cause the light to come on? This is my first vehicle with power brakes so I never had to mess with the booster before.

When I put the master cylinder on I did not measure or adjust the length of the push rod arm. If this is out of adjustment, say too long, could this cause the break light to come on intermittently?

By the way, I still hear the vacuum leak noise when driving.

Thanks for the help,

Jeff

 

The Brake warning light keeps coming on because the metering valve in the brake proportioning unit isn't centered.

You hear the vacuum because of the UN-sealed brake booster,which is supposed to be a SEALED unit to operate correctly.

Dennis
Please Don't Ask Me Any Tough Question As:
"I'm Saving Up MY Memory For When I Develop Alzheimer's"







78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles